The striatal complex constitutes the foundation of the forebrain of mammals and is believed to present an evolutionary inheritance from mammal-like reptiles. The present project is part of a continuing study that is testing the hypothesis that the striatal complex plays a basic role in the organized expression of species-typical behavior. In the current experiments focusing on tegmental structures and pathways associated with the striatal complex, it has been found that bilateral lesions of the central tegmental tract in the midbrain leads to a fragmentation and eventual elimination of the species-typical mirror displays of squirrel monkeys. In some cases, tegmental near the dienmesencephalic junction have been complicated by altered vocalization. In one animal, for example, the sound spectrogram showed that the isolation peeps had an infantile pattern. BIBLIOGRAPHIC REFERENCES: MacLean, P. D.: On the evolution of three mentalities. In Arieti, S. and Chrzanowski, G. (Eds.), New Dimensions in Psychiatry: A World View, Volume II, New York, John Wiley & Sons, pp. 305-328, 1977. Also to appear in Washburn, S. L and McCown, E. R. (Eds.), Perspectives on Human Evolution, Volume 4, Menlo Park, California, W. A. Benjamin/Staples Press, (in press).